Velocity Measuring System

ABSTRACT

A laser Doppler velocimeter uses self-mixing amplification of backreflections from scatterers below the surface of a flow. A time domain signal is divided into segments that are roughly equal to a transit time of particles through a focus of a laser beam. The segments are connected to a frequency domain through the use of an FFT algorithm to produce frequency domain data segments. Signal-to-noise ratio is enhanced through signal processing techniques using the segments to produce a final enhanced signal spectrum.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority the benefit under 35 U.S.C. §120to U.S. application Ser. No. 12/800,750 filed May 21, 2010. Said U.S.application Ser. No. 12/800,750 is hereby incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatuses for measuring theaverage velocity of an open channel flow using electromagnetic radiationsuch as, for example, laser signals to determine velocity by Dopplershift techniques without physically contacting the flow but measuringthe flow below the surface.

In one class of Doppler shift remote sensing of the velocity of an openchannel flow, a laser beam is transmitted to a flow from above and thebackscatter received from scatterers in the fluid such as bubbles, solidobjects (e.g. debris) or microscopic particles (e.g. colloid) causingturbidity, carried by the flow are sensed. The Doppler shift infrequency between the transmitted signal and the returned signal is usedto determine the velocity of the portion of the flow sampled by thelaser beam. In one embodiment, the average volumetric flow rate of thesample is determined by combining the average velocity of the flowstream measured by the laser Doppler velocimeter with other data such asthe height of the flow in the channel and the geometry of the channel.In another embodiment, the average flow velocity is determined bymeasuring the velocity of the flow at multiple locations across the flowand combining the readings to arrive at an average.

In one prior art Doppler-shift flow meter in this class, a lasertransmits a signal to the surface of a flowing stream where objects onthe surface reflect signals back. The Doppler shift between thetransmitted and reflected light is used to determine the velocity of thesurface of the flow. The localized mean velocity is calculated from thesurface velocity while the average velocity of the entire flow iscalculated from the mean velocity using the level of the flow and thecross section of the stream bed. A system of this type is disclosed inU.S. Pat. No. 5,811,688. This technique has the disadvantages of beinginaccurate under some circumstances due to the difficulty in accuratelyarriving at the mean flow velocity from the surface velocity, and ofdetecting a signal when there are few suitable reflectors on thesurface.

In still another velocity measuring, Doppler-shift, prior art technique,frequency modulated laser beams are transmitted to a target from a laserdiode and the velocity of the target is determined from the Dopplershift of harmonic frequency reflected signals and the transmittedsignals. This technique is disclosed in U.S. Pat. Nos. 6,885,438 and7,061,592. This prior art is taught only in connection with solidtargets with a focal point on the surface of the target and thus doesnot relate to some of the unique problems associated with measuring openchannel flows.

In still another prior art type of fluid velocity measuring technique,self-mixing and self amplifying laser diodes transmit beams to twospaced apart focal points within the flowing stream. Flow velocity ismeasured by the time it takes for fluid to move between the two points.This technique relies on the identification of unique signatures withinthe flow. The technique is described in “Low Cost Velocity Sensor Basedon the Self-Mixing Effect in a Laser Diode”, Opto-Electronics Review11(4), 313-319 (2003) and in “A Simple L2F Velocimeter Based onSelf-Mixing of Laser Diodes”, 14^(th) Int Symp on Applications of LaserTechniques to Fluid Mechanics, Lisbon, Portugal, 0710 July, 2008. Whilethese methods do not use Doppler shift, one of them mentions thatself-mixing diode lasers may be used in Doppler shift velocimeters.

In still another fluid velocity measuring Doppler-shift prior arttechnique, two laser beams are caused to intersect at a point in theflowing stream and the velocity at that point is determined by theDoppler shift of the scattered light. This technique is disclosed inU.S. Pat. No. 4,026,655. This patent describes the use of this techniquein measuring air speed and does not apply it to measuring velocity in anopen channel flow carrying reflecting objects.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide atleast one of:

A novel apparatus for remotely measuring the direction and flow rate ofliquid in an open channel flow with high resolution and precision.

Novel signal processing techniques that reduce noise substantially belowthe signal level. These algorithms function even though there is nophase coherence between signals returning from separate individualscatterers in the flow passing through the focus of the laser beam.

A method to reduce the contribution from surface-reflected signals in aflow meter.

A novel method for remotely classifying the turbidity of an open channelflow.

A novel technique for avoiding undesirable laser mode-hop instabilitiesin a velocimeter that can occur at particular temperatures. Suchmode-hop instabilities dramatically reduce laser coherence length whichcan render the sensor inoperable.

A novel technique to distinguish between stable and unstable laseroperation based on examination of the Doppler beat frequency spectrumnoise levels.

A method for identifying the direction of the flow.

A method for autofocusing the beam within the flow based on flow heightmeasurements from an integrated sensor, such as an acoustic sensor.

A method for efficiently coupling the laser beam energy into and out ofthe flowing medium.

A laser velocimeter which is robust to environmental effects such asmoisture condensation on the optics.

A laser velocimeter which has low power consumption for long-term use atremote locations.

A laser velocimeter that measures velocity at multiple locations acrossthe flow in order to provide a more accurate volumetric flowmeasurement.

A novel apparatus and method for measuring the flow rate of liquid.

A novel apparatus for remotely determining the direction of flowingliquid.

Novel signal processing techniques that reduce noise substantially belowthe signal level.

Novel algorithms for detecting signals related to the flowing liquidsthat function even though there is no phase coherence between signalsreturning from separate individual scatterers in the flowing stream.

A novel technique for avoiding undesirable laser mode-hop instabilitiesin a velocimeter.

A novel technique for preventing mode-hop instabilities fromdramatically reducing laser coherence length.

A novel technique for preventing mode hop instabilities which can renderthe sensor in a velocity meter inoperable.

A novel technique to distinguish between stable and unstable laseroperation.

A novel technique for distinguishing between stable and unstable laseroperation from an examination of the Doppler beat frequency spectrumnoise levels.

A method for autofocusing the beam within the flow based on flow heightmeasurements.

A laser velocimeter which is robust to environmental effects.

A novel method for remotely classifying the turbidity of a body ofliquid.

A laser velocimeter which is robust to environmental effects.

A novel synergistic relationship between the use of coherent radiationand correlation signal processing techniques to measure flow velocity.

A novel technique using coherent radiation and correlation signalprocessing techniques to obtain Doppler shift information for measuringflow rate.

A novel method for using self-mixing amplification of backreflectionsfrom scatterers below the surface of a flow for enhanced detection in alaser Doppler velocimeter.

In accordance with the above and further objects of the invention, anapparatus for measuring the velocity of an open channel flow of liquidincludes a laser diode and an optical system positioned and constructedto focus light from the laser diode to a predetermined focal pointwithin the stream. With this arrangement, a portion of the laser lightwithin the beam focus is backscattered from particulate matter in theflow, travels back along the same path as the emitted light, and isfocused back into the laser cavity. This backscattered light, whosefrequency has been Doppler shifted by its interaction with movingparticles in the flow, is then amplified and mixes with the light in thecavity emitted by the laser. Provided that the backscattered lightremains coherent with the laser light in the cavity, it produces anintensity modulation in the intensity of the output beam at the Dopplerbeat frequency. The depth at which signal data is taken is controlled bythe positioning of the focus of the laser beam in the flow since byreciprocity only the Doppler shifted backreflection from the flow at thefocus is efficiently coupled back into the laser cavity. A digitalsignal processor (DSP) is connected to the mixing system to convert theDoppler beat signal into a Doppler beat frequency spectrum. Followingthe DSP, a microprocessor calculates the speed and direction of the flowat the focal point from the Doppler beat frequency spectrum. Finally,the microprocessor is also programmed to remove any Doppler signalspikes (most probably from surface reflections) that are anomalouslystrong compared to the average Doppler signals coming from scattererswithin the bulk of the flow.

The depth at which signal data is taken in the flow can be controlledvia positioning of the focus of the laser beam since a laser Dopplersignal is only collected from the focal region of the beam. Becausereflections from the surface may not accurately represent the velocityof the bulk flow, the focus is projected into the flow so that signalsfrom features on the surface are greatly reduced. Production of aDoppler signal requires optical coherence of the backscattered signalacross much of the surface of the collection lens. With scattered light,such large-scale optical coherence upon the lens is only produced bylarge laser speckles. Such large speckles are produced only byscatterers in the focus of the laser beam. Since the optical phases ofindividual speckles projected from the same scatterer vary randomly,when numerous speckles fall on the collection optic, to first order,their beat signals in the laser cancel out. Thus, very little beatsignal appears from scatters located outside the beam focus. Largespeckles are specific to scatters located in the focal region of thebeam; laser speckles projected from scatterers in other beam regionswill be quite small, projecting many speckles across the collection lenswhich result in the production of a negligible laser Doppler signal.Finally, the microprocessor is programmed to remove any Doppler signalspikes (most probably from surface reflections) that are anomalouslystrong compared to the average Doppler signals coming from scattererswithin the bulk of the flow.

One embodiment of the optical system consists principally of acommercially packaged laser diode and an external lens. Within theoptical cavity of the laser a diode junction emits highly coherent laserlight. The coherent laser light diverges out of the optical cavitytowards the external lens. The divergence angle of the beam out of thelaser package is set by either the fabrication of the laser diodeoptical cavity or by a corrective lens embedded inside the laserpackage. The f-number of the external lens is chosen to match thedivergence angle of the coherent light from the laser in order tooptimize light collection from the laser. The position of the externallens is chosen to gather as much of the divergent laser light from theoptical cavity as possible. The position of the external lens is furtheradjusted to focus the laser light below the surface of the flowingstream. The position of the focal point in the flow is determined by thedistance between the laser diode optical cavity and the external lensaccording to the lens maker's formula. A small amount of the laser lightin the focal region is Doppler shifted as it is scattered back into thelaser cavity from particles moving along in the flowing stream. Sincethese particles are flowing with the stream, their velocity isrepresentative of the velocity of the flowing stream at the focal point.By reciprocity, light scattered from particles within the beam focus inthe stream which falls within the acceptance cone of the external lensis coupled back into the laser diode optical cavity by the externallens. This effect aids in discriminating against light scattered fromelsewhere in the beam, such as from the surface of the flowing stream.If the Doppler shifted backscattered light remains coherent with thelight in the laser cavity the net intensity out of the laser ismodulated at the Doppler beat frequency. A window in the optical path,that serves to protect both the laser and the external lens from theenvironment, must be of sufficient optical quality, e.g. flatness, thatit preserves the spatial quality of the beam for efficient operation ofthe optical system.

To increase the amount of light transmitted to scatterers below thesurface, the laser diode is oriented to produce p-polarized light (inthe plane of reflection) to make use of Brewster's angle to reducesurface reflections as the beam enters the flow. Still, even with a beamfocused in the flow, commonly the raw Doppler signal seen by thephotodiode in the laser velocimeter will be below the level from noisesources, such as shot noise due to photodiode current. Overcoming thislimitation was key to producing a functional Doppler flow sensor. Signalprocessing techniques are employed to recover a usable Doppler signaturefrom within this noise. The Doppler signature arises as a series ofshort pulses, or bursts, produced by scatterers in the flow which passthrough the focus of the optical beam. The phase of the Doppler signalvaries randomly with each new scattering particle. For processing, theDoppler signal is transformed from the time domain to the frequencydomain with a Fast Fourier transformer. The input signal is sampled at arate that is high enough to accommodate the Nyquist limit of the Dopplerfrequency from the highest flow rate to be measured. However, even whenthe signal from the photodiode is subjected to a Fast Fourier Transform(FFT), there is no boost in signal-to-noise ratio since the signals fromthe scatterers are incoherent with each other and the signal itself isoften lost in the vast amount of background noise between Dopplerbursts. In one embodiment, the signal-to-noise ratio is increased byprocessing the signal as a series of short duration FFTs. The durationprocessed by each FFT is 1× to 2× longer than the transit time of aparticle through the focus of the laser beam. The frequency spectra fromall of the FFTs are then averaged together, resulting in a spectrum withan increased signal-to-noise ratio. This resulting averaged spectrum isthen used as the final signal.

After a background spectrum is created by defocusing the beam within theflow, pattern recognition algorithms first eliminate peaks common to thesignal and background spectra and then examine the remaining peaks toidentify the relevant peak in the averaged FFT corresponding to thefluid flow velocity, through the elimination of features such as spikesin the spectra that could be due to surface reflections. With knowledgeof both the liquid level and the open channel conduit geometry, whichprovide the cross-sectional area of the flow, the expected flow rate canbe calculated by multiplying the measured velocity value and the crosssectional area of the flow. This value may be further refined bymodeling the relationship between the measured velocity value and theaverage flow velocity.

In another embodiment to determine turbidity, the system microprocessorincludes a program that changes the focus point between one depth andanother. The variation in the strength of the backscatter signal as thedepth is increased correlates with the turbidity of the liquid. Thehigher the rate of attenuation of backscatter with respect to depth, thegreater the turbidity. The principle behind this technique has somesimilarities to a Secchi disk. A Secchi disk is lowered into a naturalbody of liquid until its image is no longer visible due to attenuationand scattering of the light from its image. The distance below theliquid surface, known as the Secchi depth, decreases with higher ratesof attenuation due to higher levels of turbidity. The microprocessor maybe calibrated to provide a scale of turbidity.

To avoid prolonged mode-hop laser instabilities, which arise from slowchanges in the temperature of the laser diode, from preventing datacollection, in one embodiment the microprocessor causes power to beapplied to the laser diode only for short periods of time during whichbeat signal measurements are taken. Immediately after the laser diode isturned on its temperature changes rapidly for several seconds. Duringthis time the laser tunes rapidly through several stable and unstableoperating regions. During analysis data collected during stableoperation is processed while that taken during unstable operation isdiscarded. An added benefit of turning on the laser diode for shortperiods of time is that it reduces the duty cycle for sensor operation,thereby increasing power efficiency and yielding longer batterylifetimes in remote sensor operations. An alternate method of operationplaces a small heater band around the laser diode package to shift thetemperature of the diode to a stable operating point where it can bemaintained without significantly compromising power efficiency.

In a further development of this embodiment, to determine the directionof the flow the laser diode is mounted on a fixture that can be ditheredback and forth by less than a millimeter along the direction of thelaser beam. Movement of the laser along the beam path, with a projectionpointing in the same direction as the flow, lowers the observed Dopplerbeat frequency while movement of the laser, with a projection in adirection opposite the direction of flow, increases the observed Dopplerbeat frequency (see FIG. 8 for a full discussion of this process).

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the present disclosure. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate subject matter of the disclosure.Together, the descriptions and the drawings serve to explain theprinciples of the disclosure.

DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1 is a simplified schematic diagram of a system for remotelymeasuring the flow rate of a fluid stream;

FIG. 2 is a schematic block diagram of the optoelectronics used in theflow meter of FIG. 1;

FIG. 3 illustrates the positioning of a laser Doppler velocimeter beamwaist of diameter D in a flow of nominal velocity, V where the transittime of particles through the beam waist is on the order of time, T=D/V.

FIG.4 shows a representative time trace that might be produced by thesystem of FIG. 1. Occasional Doppler “bursts” are generated byscatterers passing through the beam waist amidst long periods ofbackground noise where no scatterers are detected.

FIG. 5 illustrates the typical frequency spectrum produced when the FFTis evaluated over long (in comparison to particle transit times throughthe beam) time intervals.

FIG. 6 pictures a time trace similar to that shown in FIG. 4 except thatit is being divided into time intervals that are roughly comparable tothe transit time of particles through the beam waist.

FIG. 7 illustrates FFTs taken over each of the short time intervalsshown in FIG. 6. When summed together noise in the resultant FFT isreduced by an amount proportional to the square root of the number ofsamples being averaged.

FIG. 8 illustrates the process by which the direction of flow isdetermined by dithering (+/−) the position of the laser diode during aDoppler measurement.

FIG. 9 is a flow diagram of the preferred embodiment of a process orcomputer program for determining the volumetric flow rate of a flowingstream.

FIG. 10 is a flow diagram of a subroutine of a process of FIG. 9 ofcollecting a background spectrum.

FIG. 11 is a flow diagram of a subroutine of a process of FIG. 9 ofstepping the laser to a position with respect to the lens that causesfocusing at the predetermined distance beneath the surface of the flowstream for automatic focusing.

FIG. 12 is a flow diagram of a subroutine of a process of FIG. 9 ofcollecting a Doppler signal spectrum.

FIG. 13 is a flow diagram of a subroutine of a process of FIG. 9 offinding a valid Doppler peak in the spectrum.

FIG. 14 is a flow diagram of a subroutine of a process of FIG. 9 ofcalculating the unsigned Doppler flow velocity.

FIG. 15 is a flow diagram of a process within the process of FIG. 9 ofdetermining the sign direction of the flow.

FIG. 16 is a flow diagram of a subroutine of a process of FIG. 9 ofcalculating the average volumetric flow.

FIG. 17 is a flow diagram of a subroutine for profiling a flow stream.

FIG. 18 is a top view of a sketch of the basic optical system showinghow shifting the laser diode sideways (perpendicular to the beam path)also changes the position of the focal point within the flowing streamfrom side-to-side.

FIG. 19 is a side view of the sketch in FIG. 18.

FIG. 20 is a top view of a sketch of the basic optical system showinghow shifting the laser diode along the light beam axis also changes theposition of the focal point within the flowing stream along the beamaxis.

FIG. 21 is a side view of the sketch in FIG. 20.

DETAILED DESCRIPTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

In FIG. 1, is shown a schematic block diagram of a flow meter forremotely measuring the average velocity of an open channel flow, 30. Inthis specification the words “open channel flow”, “flow”, “liquid flow”,“rate of flow” and “flowing steam” are used from time to time todesignate the medium for which the flow meter is used. These words arenot intended to be words of limitation as to the nature of the liquid,the rate of which is being measured, and the novel flow meter and areinterchangeable when designating the medium characteristics of which arebeing measured. The flow meter 10 includes as its principal parts aDoppler beat frequency subsystem 32, a laser diode module 36, amicroprocessor subsystem and input/output devices 70, a lasertemperature and flow stream level monitoring system 14, and aenvironmental protection system 22. The flow meter uses a laser diodemodule 36, that contains a laser diode emitter 78 and a photodetector 80mounted behind the rear facet of the laser. With this combination ofparts, the laser diode module 36 transmits a p-polarized beam 74 throughthe optical system to a location or focus point 28 within the flowstream 30.

The Doppler beat frequency processing subsystem 32 includes analogprocessing circuits 82, an A/D converter 60, a digital signal processor62 and a laser temperature monitor 84. The digital signal processor 62and the laser temperature monitor 84 are in electrical communicationwith the microprocessor subsystem and input/output devices 70 to supplysignals indicating the Doppler beat signal and the laser temperature tothe microprocessor subsystem and input/output devices 70. These signalscan be used by the microprocessor subsystem and input/output devices 70to correlate the temperature of the laser diode with stable and unstableperiods of operation of the laser diode and to provide information tothe operator through the output devices within the microprocessorsubsystem and input/output devices 70. The analog processing circuits 82are in communication with a photodetector 80 within the laser diodemodule 36 to receive Doppler signals and with the ND converter 60 toconvert the signals to digital form and transmit the digitized Dopplersignals to the digital signal processor 62 for FFT evaluation beforebeing sent to the microprocessor subsystem and input/output devices 70for further processing.

To avoid operating temperatures at which the diode laser experiencesprolonged mode hop instabilities data is taken immediately after thelaser is turned on. As the laser approaches thermal equilibrium itpasses through several regions of stable and unstable operation. Datarecorded during this period is examined by the microprocessor 70 which,based on the characteristics of the noise levels, distinguishes betweenstable and unstable operation. After the laser reaches thermalequilibrium and the measurement is done, it is turned off until the nextmeasurement cycle. Periodic measurement cycles reduce the sensor dutycycle, thereby reducing battery drain, saving power, and enabling longerunattended remote operation for the sensor.

The beam focusing system 14 includes a level sensor 34 and an autofocusdriver 40 each of which communicates with the microprocessor subsystemand input/output devices 70. The microprocessor subsystem andinput/output devices 70 includes a program that controls the autofocusdriver to position the focus of the beam at a predetermined depth thatmay be recorded in memory. In this specification, the words“predetermined depth” mean the depth of the focus of the transmittedlaser beam. The predetermined depth is chosen in accordance with thepurpose of the measurement. However, in any case, the focus is at adepth sufficient to avoid spurious contributions to the velocitymeasurements from the surface and close enough to the surface of theflowing stream 30 so that p-polarized light of significant intensityreaches light scatterers below the surface. Such backscattering objectsare referred to herein as backscatter material and may include anymaterial that backscatters light including for example colloidalsuspensions as well as solid material. The words, “significantintensity” mean an intensity that permits measurement of velocity of themovement of the flowing stream within a reasonable margin of error.Hereinafter, the depth of the focus that meets the above criteria isreferred to as a predetermined depth. The reasonable margin of errordepends on the use to be made of the measurements. The meaning of thewords “reasonable margin of error” in this specification depends on theuse to be made of the measurements. It is an error that permits theintended use of the measurement, e.g. to determine if a flow of liquidwill overload a sewer system.

The beam from the laser is automatically focused to a point 28, belowthe surface of the flow by a lens 48, attached, in one embodiment of thesystem design, to an autofocus driver 40, whose movement is controlledby the microprocessor 70, based on level measurements provided by aconventional level sensor 34, such as an ultrasonic time-of-flightsensor. Focusing below the surface effectively eliminates spuriouscontributions to the velocity measurement from surface reflections. Inthis specification, the words “spurious contributions to the velocitymeasurements” mean that reflections from light scatters on the surfacecan give inaccurate velocity readings and are hence removed from thedata used for velocity calculations. To maximize the amount of lighttransmitted below the surface, the linearly polarized laser diode isoriented to make use of the Brewster's angle reflection property forp-polarized light 74 as it enters into the flow. In this specification,the words, “Brewster's angle” shall mean substantially at the Brewster'sangle and the words “substantially at the Brewster's angle” shall meanat an angle permitting minimal reflection loss at the surface of theflow. As an example, for <1% surface reflection from p-polarized lightincident on flowing liquid Brewster's angle is in the range of 42 to 63degrees.

In the preferred embodiment laser light that is Doppler shifted andbackscattered from particles moving in the flow 30, returns to the laser78, where it mixes with the original laser beam to produce a Dopplerbeat signal that is picked up by the photodetector at the rear facet ofthe laser in a self-mixing process that was first described in “LaserDoppler velocimeter using the self-mixing effect of a semiconductorlaser diode”, Appl. Opt. 25, 1417-1419 (1986). To determine thedirection of the flow the laser diode is dithered 46 along the beam pathduring a measurement (see FIG. 8 for a detailed description). A numberof methods, such as magnetic or piezoelectric approaches, can be used todither the very small laser diode unit.

In this embodiment the analog Doppler signal is processed by the AnalogProcessing Circuits 82, and is then converted from analog-to-digital byan A/D Converter, 60. The signal-to-noise ratio of the Doppler beatspectrum is enhanced by the square root of the number of samples bysumming together a series of Fast Fourier Transform (FFT) spectraproduced by the digital signal processor (DSP) 62 from dividing the timetrace into sequential time segments whose duration corresponds roughlyto the transit time of scattering particles in the flow passing throughthe beam focus (see discussion of FIG. 3). Next, a background spectrumis created using the same process outlined above but with the beam inthe flow defocused. Pattern recognition algorithms in the microprocessor70, first eliminate peaks common to the signal and background spectraand then examine the remaining peaks; comparing their linewidths, shapesand locations with prior recorded flow data to help identify the peak inthe averaged FFT that corresponds to the bulk flow rate. Knowledge ofthe open channel conduit geometry and flow level, used with the flowrate model, can provide an estimate of the flow velocity that will aidin finding the Doppler signal peak in the spectrum.

To protect the optics from the environment several technologies can beemployed. A tube 66 whose inner surface is coated with a hydrophilicmaterial intercepts moisture entering the tube and draws it away fromthe glass window 86 so that it runs back down the wall of the tube andout the open end while a short air blast from a compressed air source76, clears any remaining condensation or solid debris from the opticsbefore collecting data. The flat optical quality window mounted at theBrewster's angle 86 protects the sensitive and expensive focusing optics48 and focusing mechanism 38 from moisture and corrosive environmentalgases. Lastly, an optical or electronic shutter 52 (while the shutter 52is shown as a mechanical shutter, in practice it should have speed ofopening and closing that is sufficient to provide the high sampling raterequired and will be an optical or electrical shutter) at the end of thecoated tube 66 is only open for the brief periods during which data iscollected to further limit exposure of the optics to the outsideenvironment.

In addition to providing basic flow measurement this device can beconfigured to provide several other useful bits of information about theflow. By collecting data sequentially at different depths the turbidityof the flow can be estimated from the change in intensity of thebackscattered Doppler signal with depth. Volumetric flow in the openchannel can be extrapolated from a single point velocity measurement ata known location in the flow given a knowledge of flow height in thechannel and channel geometry. Volumetric flow can be more accuratelydetermined by measuring the velocity at multiple locations within across-section of the flow.

In FIG. 2 is a schematic block diagram of the basic optoelectroniccomponents found in one preferred embodiment of the current inventionfor remotely measuring average flow in an open channel. The devicecontains a self-mixing laser subsystem 24, an optical subsystem 26, aDoppler beat frequency processing subsystem 32, a level sensor 34 and amicroprocessor subsystem 70.

FIG. 2 also includes a schematic drawing of the Doppler shift frequencyprocessing subsystem 32 shown in FIG. 1 as a block diagram. It includesa preamplifier 54, a blocking capacitor 56, an RF pre-amplifier 58connected to an analogto-digital converter 60 which in turn is connectedto a digital signal processor 62. Signals from the laser diode 36generated in response to backreflected light are passed through a bufferamplifier 54 before being AC-coupled through blocking capacitor 56 tothe input of an RF pre-amplifier 58. The pre-amplifier 58 applies thesignal through voltage dropping resistor 72 to the analog-to-digitalconverter 60. Signals from the analog-to-digital converter 60 areapplied to the digital signal processor 62 which performs a series ofFFT operations on the data and then sends the resulting spectrum to thememory 64 and to the microprocessor unit 68 within the microprocessorsubsystem and input/output devices 74.

The self-mixing laser subsystem 24 includes the laser diode module 36,the lens position adjustment mechanism or screw drive 38, the focusdrive motor 40, the laser driver constant light power unit 42 and thepower drive 44. The laser diode module 36 transmits light to the focusand receives backscattered light. It mixes the transmitted lightfrequency and the backscattered light and provides a Doppler beat signalto the Doppler beat frequency processing subsystem 32 to which it iselectrically connected. The lens position adjustment mechanism 38 movesthe diode module with respect to the optical subsystem 26 to focus thebeam of light. The power drive 44 receives signals from themicroprocessor subsystem and input/output devices 70 that sets thelocation or series of locations of the focus and applies the requiredpower to the focus drive motor 40 to which it is electrically connectedto position the laser diode module 36 accordingly.

During a flow measurement the microprocessor communicates with the levelsensor 34 to determine the flow level in the open channel. In thisembodiment the microprocessor commands the focus drive motor 38 toposition the diode laser module 36 to focus the laser beam some depthbelow the surface of the flow. Next the shutter 52 is opened, an airblast 76 clears the flat optical quality window 86, the diode laser 78is powered on while several seconds of beat signal, created by Dopplershifted backscattered light from the flow mixing with laser light in thecavity, is sent by the photodetector 80 to the Doppler beat frequencyprocessing subsystem 32. In another embodiment, an ultrasonic orelectromechanical vibrator attached to the window could also be used toclear the window.

The processed Doppler frequency spectrum is then stored in themicroprocessor memory 64. The microprocessor then moves the laser module36 to defocus the laser beam in the flow and a background frequencyspectrum is stored in the microprocessor memory. Next the processorcompares the signal and background spectra to eliminate peaks common toboth spectra. The remaining peaks are examined with a customized patternrecognition algorithm in order to identify the peak in the FFT spectrumthat corresponds to the fluid flow velocity. From the effect of thedither on the identified peak the direction of flow relative to themeasuring system can be determined (see FIG. 8). Laser dithering canalso be used to identify or confirm peaks in the frequency spectrum asDoppler signal from the flow stream since their frequencies will be red-shifted (increased) or blue-shifted (decreased)accordingly; whereas,noise peaks will be unaffected.

FIGS. 3-7 illustrate a preferred method for increasing thesignal-to-noise ratio of the backscattered Doppler beat signal obtainedfrom an open channel fluid flow by self-mixing in a diode laser. Solidmaterial flow produces a continuous, coherent Doppler beat signal which,when converted to the frequency domain by a Fast Fourier Transform,yields a strong peak in the frequency spectrum that corresponds to thespeed of the material flow. However, when backscattering from particles132 in an open channel flow 30 such as that pictured in FIG. 3 aremonitored the situation is very different. A series of short Dopplerbeat signal bursts (134 in FIG. 4) are observed by the photodetector 80as particles in the flow 30 transit the beam focus 28 and backscatterlight into the diode laser. The duration of these Doppler beat signalbursts is approximately T=D/V, where D is the diameter of the beam focusin the flow and V is the speed of the flow.

The FFT of a long time sample such as that shown in FIG. 4 produces noclear Doppler beat frequency peak (FIG. 5) as either the beat signalbursts are so infrequent that their signal is lost in the backgroundnoise or the incoherent phasing of succeeding bursts leads to no netimprovement of the Doppler beat signal.

FIGS. 6-7 show one preferred method that greatly increases thesignal-to-noise ratio of a Doppler beat frequency measurement for openchannel flows. The time trace in FIG. 6 is identical to that shown inFIG. 4 except that it is divided into segments of time (139A-139J), T,that correspond approximately to the transit time of particles throughthe beam focus in the flow. Taking an FFT of each one of these shorttime segments produces a series of frequency spectra (140A-140J) thatwhen summed together produce a spectrum with a strong Doppler beatfrequency peak 143. Summing together a large number of FFTs in this wayimproves the SNR of the resultant FFT by an amount that is proportionalto the square root of the number of FFTs in the summation. Furthermore,in this process the incoherent phasing of signals from differentscatterers does not act to degrade the resultant FFT. An additionalimprovement in SNR can be made by eliminating any segments that haveunusually large amounts of noise such as those created when the laser isbecoming unstable prior to mode hopping.

FIG. 8 illustrates how adding a small (<1 mm) back-and-forth motion(dither) to the laser diode along the beam path permits one to determinethe direction of fluid movement in an open channel flow. When a Dopplershifted, backreflected beam from particles in the flow moving towards(102) or away (104) from the apparatus is mixed with the original laserbeam frequency (100) in the laser cavity a Doppler beat signal fp isproduced. If the beat signal is very strong it can be observed directlyin the time domain as a sawtooth waveform whose positive or negativeslope is determined by the direction of flow as described in “LaserDoppler velocimeter using the self-mixing effect of a semiconductorlaser diode”, Appl. Opt. 25, 1417-1419 (1986). However, if thebackreflected beam is very weak then the procedure outlined in FIG. 8 isrequired to determine the direction of the flow. Since only thefundamental frequency in the beat spectrum is observable, flow towardsor away from the apparatus will yield the same peak at f_(D) in the beatfrequency spectrum. Without additional information it is not possible todetermine the direction of flow from the Doppler beat frequencyspectrum. One method of determining the flow direction is to apply asmall (<1 mm) back-andforth motion (dither) to the laser diode along thelaser beam path during a measurement at a speed that is sufficient toshift the Doppler beat frequency by an easily observable amount (forexample, 10-50 kHz). The change in the Doppler beat signal is differentdepending on the direction of fluid flow. For flow towards the apparatus(102) the forward stroke of the dither increases the beat signal tof_(D) ⁺ (106) while the backward stroke of the dither decreases the beatsignal to f_(D) ⁻ (108). For flow away from the apparatus (104) theforward stroke of the dither decreases the beat signal to f_(D) ⁻ (110)while the backward stroke of the dither increases the beat signal tof_(D) ⁺ (112). A further benefit of this dithering process is that itcan be used to confirm no flow (zero velocity) conditions, whichotherwise would result in no Doppler beat signal and thus no outputsignal due to the AC coupling (blocking capacitor 56) of the electroniccircuitry (FIG. 2).

In FIG. 9, there is shown a flow diagram 144 of the preferred embodimentof a process or computer program for determining the volumetric flowrate of a flowing stream having the step 146 of collecting backgroundspectrum, the step 148 of automatically focusing the laser light belowthe surface of the flowing stream, the step 150 of collecting a Dopplerspectrum while keeping the laser emitter stationary, the step 152 offinding the Doppler peak in the above collected Doppler spectrum andreturning its corresponding Doppler frequency shift, the step 154 ofdetermining unsigned Doppler flow velocity, the step 156 of determiningsigned direction of the flow, and the step 158 of calculating volumetricflow and gathering data. Each of these steps is described in greaterdetail below.

In FIG. 10, there is shown a flow diagram of the process 146 ofcollecting a background spectrum having the step 162 of keeping theshutter closed or focusing the laser to infinity, the step 164 ofturning on the laser, the step 166 of collecting N consecutive, equallytime spaced signal data points at a sufficiently high sampling ratefrequency, the step 168 of dividing the sample into transit-time lengthsegments, the step 170 of converting the above transit-time signal datainto a set of frequency domain spectra by Fast Fourier Transform (FFT),the step 172 of rejecting poor quality spectrum and retaining goodquality spectrum that does not include spectra recorded during times oflaser instability caused by mode hopping, the step 174 of continuing toacquire more good quality spectra until K, a sufficiently large number,of good spectra has been acquired, the step 176 of summing the K goodquality spectra into a single spectrum and storing this spectrum as abackground spectrum, and the step 178 of turning off the laser. Thebackground spectrum is obtained to avoid basing the velocity readings onnoise that could otherwise be misinterpreted as a Doppler shift but isactually independent of focusing or shining the laser light into theflowing stream. The background signal contains possible electricalsignals that do not originate from backscattering of the laser lightfrom the flowing stream. The collection of background spectra avoidsmisinterpreting as signals sufficiently strong electrical signals thatare not caused by scatterers in the flow stream, and thus should beconsidered noise.

In FIG. 11, there is shown a flow diagram of the process 148 ofautomatically focusing the laser light in a predetermined distancebeneath the surface of the stream having the step 180 of measuring andrecording the height or depth of the flowing stream using some form oflevel sensor, such as an ultrasonic time-of-flight sensor, the step 182of using the angle value, typically but not limited to 45°, between thelaser beam axis and the flowing stream level, the above stream heightvalue from step 180 and information about the pipe-channel geometry tocalculate the distance between the lens and the desired focal pointwithin the flowing stream, the step 184 of using the Lens Maker'sFormula and focal length of the lens to calculate the required distancebetween the laser emitter and the lens, the step 186 to home theposition of the laser emitter and the step 188 of positioning the laseremitter the required distance away from the lens. The stream heightvalue from step 180 is recorded and used elsewhere in the datacollection and analysis. In the preferred embodiment, the laser emitteris moved in order to adjust the required distance between the lens andlaser; however, for the purpose of focusing, the lens could be movedalso to adjust this required distance. Furthermore optical-qualitymirrors could be inserted between the laser emitter and lens to adjusttheir effective optical distance, but this would add further complexityto the system.

In FIG. 12, there is shown a flow diagram of a process 150 forcollecting signal and spectrum comprising the steps 190 of opening theshutter, clearing the window and turning on the laser, the step 192 ofcollecting N consecutive, equally time-spaced signal data points at asampling rate that is high enough to accommodate the Nyquist limit ofthe highest Doppler frequency to be measured, the step 194 of dividingthe sample into transit-time length segments, the step 196 of convertingthe transit-time signals into frequency domain spectra by Fast FourierTransform (FFT), the step 198 of rejecting poor quality spectra andretaining good quality spectra that does not include measurements takenduring laser instabilities caused by mode hopping, the step 200 ofcontinuing to acquire more signals and spectra until K number, i.e. asufficiently large number, of good quality spectra have been acquired,the step 202 of summing the K good quality spectra into a singlespectrum and storing this spectrum as a Doppler spectrum, and the step204 of turning off the laser and closing the shutter. In this fashion,the signal is collected during periods of time when the laser is stableand not hopping from mode to mode. To further enhance thesignal-to-noise ratio, the step 198 could also reject spectra, that havebeen determined not to contain any contributions from theDoppler-shifted, backscattered laser light. In this process, the laseremitter is kept stationary with respect to the lens.

A collected spectrum should be rejected based on its overall noiselevel. A spectrum collected during unstable operation of the laser has asignificantly higher noise level than a spectrum collected during stablelaser operation. In general, spectra collected during stable laseroperation have minimum noise levels. The noise level of a spectrum canbe estimated by averaging the baseline values at several frequencies inthe spectrum other than at the frequency of the Doppler shift. The noiselevel of a spectrum can also be estimated by summing together the poweror intensity values for each frequency over the entire spectrum, sincethe Doppler signal is small compared to the total noise. Finally thepower current drawn by the RF preamplifier (FIG. 2, 58) during thesignal acquisition of a spectrum is also indicative of the noise levelof that spectrum. In this specification or claims, the words “noiselevel floor” may be used from time to time. In this specification, thesewords mean the strength of the signal created from the sum of all thenoise sources and unwanted signals. While the noise floor and peaks areused to distinguish between unstable and stable signals, othercomparisons are possible such as the RMS of the noise and the peak noisesignals.

In FIG. 13, there is shown a flow diagram of the process 152 ofidentifying the peak in the spectrum obtained in process 150 due to theDoppler beat signal having the steps 210 of using the previous velocitymeasurement or the result from a Flow modeling equation, to aid insearching the Doppler velocity spectrum for a peak within theneighborhood of the estimated velocity value bound by a set of tolerancebands, or the step 212 of searching the Doppler spectrum directly usinga first-derivative slope filter, such as a Savitzky-Golay filter or anysuitable finite-difference technique, for the occurrence of the spectralcurve slope increasing above a predetermined slope threshold valuesetting, indicating the start of a peak in the spectral curve, the step214 of ensuring that the peak found in either step 210 or step 212 has awidth at least as wide as the predetermined minimum peak width valuesetting, the step 216 of ensuring that the peak does not also occur inthe background spectrum obtained in process 146 (FIG.10), the step 218of continuing to search for another peak in the Doppler spectrum byeither step 210 or step 212, if the first peak does not satisfy thewidth criterion or is also found in the background spectrum, the step220 of taking the peak frequency as the Doppler frequency value when avalid peak has been found; otherwise, repeat the Doppler spectrumcollection described in process 150, and finally the step 222 ofreturning the value of the Doppler frequency.

In FIG. 14, there is shown a flow diagram of the process 154 ofcalculating the unsigned Doppler flow velocity value from the Dopplerfrequency value returned from process 152 having the steps 230 of usingthe angle value with respect to the flowing stream level and the Dopplerformula to calculate the velocity of the flowing stream from the Dopplerfrequency value, and the step 232 of returning the Doppler velocityunsigned value as the magnitude of the velocity for the flowing stream.

In FIG. 15, there is shown a flow diagram of a program 156 whichprovides the software processes for determining the sign, or thedirection of flow, of a flowing stream. This program includes theprocess 240 of automatically focusing the laser light into the flowstream below the surface at a predetermined level, similar to process148 (FIG. 11), the process 242 of collecting a Doppler spectrum whilemoving the laser emitter towards the lens, similar to process 150 (FIG.12) which instead keeps the Laser emitter stationary, the process 244 offinding a Doppler peak in the “Toward” spectrum, similar to process 152(FIG. 13), the process 246 of automatically focusing the laser lightagain, since the laser emitter was moved in process 242, the process 248of collecting a Doppler spectrum while moving the laser emitter awayfrom the lens, the process 250 of finding the Doppler peak in the “Away”spectrum. The “Toward” spectrum is a Doppler spectrum that has beencollected while moving the laser emitter towards the Lens; whereas, the“Away” spectrum is a Doppler spectrum that has been collected whilemoving the Laser emitter away from the Lens. Typically the Dopplerspectrum is collected while the Laser emitter remains stationary, as inprocess 150 (FIG. 12), in determining the unsigned Doppler flow velocityvalue. The descriptors “Toward” and “Away” are also used forcorresponding values, such as the Doppler frequency value. This program156 continues with the steps 252 of comparing the “Toward” Dopplerfrequency value and the “Away” Doppler frequency value with each otherand assigning a positive value to the sign direction of the flow whenthe “Toward” value is greater than the “Away” value; otherwise, itassigns a negative value to the sign direction, and the final step 254of returning the sign direction value of the flow. A positive signdirection value indicates a flow toward the velocimeter; whereas, anegative sign direction value indicates a flow away from thevelocimeter.

The principle behind this concept is based on the fact that the motionof the laser emitter, which serves as both transmitter and receiver ofthe electromagnetic wave, affects the relative velocity between thelaser and the flowing stream, which in turn affects the Dopplerfrequency. Moving the laser emitter in the same direction as the flowwill reduce their relative velocity and corresponding Doppler frequency.Moving the laser emitter in the opposite direction of the flow willincrease their relative velocity and corresponding Doppler frequency.Since the direction of the laser motion is known, the direction of theflow can be deduced from the Doppler frequency change. Assigning apositive value to flow toward the velocimeter and negative value to flowaway from the velocimeter is arbitrary.

In another embodiment of this concept, the laser emitter could bedithered towards the lens while collecting N time signal data points forthe first FFT, and the laser emitter could than be dithered away fromthe lens while collecting N time signal data points for the second FFT.The time duration needed to sample N time signal data points at anadequate sampling rate would be short. The laser would only move a veryshort distance for each dither, and thus would not change the focalpoint significantly. Also with the laser emitter being dithered back andforth for alternating prepared data segments, the laser need only beautomatically focused once. Furthermore, the laser emitter could bemoved by a piezoelectric or a magnetic translator.

In another concept, the direction of the flow may also be determined byrapidly increasing or “chirping” the emitted laser light frequencyinstead of physically moving the laser emitter. A rapid increase or“chirp” in emitted laser light frequency can be achieved by eitherballisticly heating the laser diode or rapidly increasing (ramping) itselectrical driving current. Initially the emitted laser light has afrequency f₀. Since the laser diode is being chirped, the laserfrequency will be f₀+d by the time the Doppler-shifted, backscatteredlight returns. This returned light will either have a frequency of f₀+wfor flow toward the laser emitter and velocimeter or a frequency f₀−wfor flow away from the laser emitter and velocimeter, where w is theDoppler frequency shift. This returned light will now mix with the newlaser frequency f₀+d giving either a beat frequency d+w for flow towardthe velocimeter or a beat frequency d−w for flow away from thevelocimeter. As long as the value d is larger than the value w, thevalues d+w and d−w will both be positive. The offset frequency dresolves any ambiguity due to measuring only the magnitude of thevelocity.

Both concepts are intended to resolve the flow direction of the stream;however, both concepts can be used to confirm a no flow condition. Inthe case of no flow, the stream velocity would be zero and thecorresponding Doppler frequency shift would also be zero. Due to theelectrical circuitry of the velocimeter (e.g. AC-coupling of thesignal), a Doppler frequency shift of zero is the same as no Dopplersignal. A no signal event could be caused by either a zero-velocityliquid flow condition or no scattered light from the liquid or a breakin the circuitry. Both concepts can be used to resolve this ambiguity.

In FIG. 16, there is shown a flow diagram of the process 158 ofcalculating the volumetric flow having the step 260 of calculating thecross-sectional area of flowing stream using the stream height value andknowledge of pipe geometry, the step 262 of calculating the volumetricflow value from the cross-sectional flow area, the unsigned Doppler flowvelocity value and the signed Direction of the flow, the step 264 ofdetermining the average volumetric flow value from use of an industryaccepted method such as the average velocity being 90 percent of themaximum velocity value as measured near the surface of the flowingstream or profiling the stream at various depths and locations, forminga grid of focal points, below the surface of the stream by eithershifting the laser focal point toward and away from the lens along theaxis of the light beam (FIGS. 20 & 21) or shifting the laser focal pointperpendicularly from the light beam, side to side, (FIGS. 18 & 19) andthe step 266 of using the above average volumetric flow value tooptimize the flow modeling equation parameters at a particular site forbetter estimates in future measurements as in a learning process.

In FIG. 17, there is shown a flow diagram of a process 270 for using thelaser light backscattered from the focal point or points below thestream surface with reduced light reflection from the surface of theflow stream to determine a rate of flow of the flowing stream from agrid of focal points or to determine the mean or average velocity of theflow stream having the steps 272 of blocking or removing at least someof the surface reflections from the signal used to determine Dopplervelocity of the flow stream, the step 274 of using the laser lightscattered back from a grid of focal points covering a substantial areato determine the average volumetric flow of the stream or the step 276of using the laser light scattered back from a single focal point todetermine a flow velocity of the flowing stream at the focal pointand/or the mean or average velocity of the flow stream and calculatingthe volumetric flow rate.

In FIG. 18, there is shown the top view of the basic optical systemillustrating how shifting the position of the laser diode (304) sidewaysorthogonal to the beam path with a translating mechanism (306) alsochanges the position of the focal point within the flowing stream (300)from side-to-side along the width of the stream. In FIG. 19, there is aside view of the same illustration in FIG. 18. In FIG. 20, there isanother top view of the basic optical system illustrating how shiftingthe position of the laser diode (304) along the light beam axis with atranslating mechanism (306) also changes the position of the focal pointwithin the flowing stream (300) along the light beam axis. In FIG. 21,there is a side view of the same illustration in FIG. 20. FIGS. 18 and19 illustrate how the laser diode should be moved to change the positionof the focal point along the width of the flowing stream so as tomeasure flow velocities at various points along the width of the stream.FIGS. 20 and 21 illustrate how to move the laser diode so as to measureflow velocities at various depths below the surface of the flowingstream. A combination of these two techniques allows the velocimeter tomeasure flow velocities over a grid of points covering a cross-sectionof the flow stream. These measurements taken over the cross-section ofthe stream can better estimate its average flow velocity or obtain itsflow velocity profile.

There are several techniques that may be used together or separately toreduce the deleterious effects of surface reflections on the calculationof volumetric rate of flow of a flow stream. One technique that could beused in software is based on the fact that surface reflections arebrighter than scattering within the bulk of the flowing stream. Alsosurface reflections are from surface bubbles, ripples or other floatingobjects. These objects are macroscopic, i.e. easily visible by theunaided human eye. Due to their larger size, they are less affected byBrownian motion or local turbulence than microscopic particles, whichcontribute to the turbidity of the flowing stream and thus serve toscatter the laser light within the bulk of the flowing stream. Surfaceobjects have a significantly narrower velocity distribution than theturbidity-causing, microscopic particles. Due to brighter reflectionsand narrower velocity spread, surface reflections give sharp, intenseDoppler peaks in the spectra. They appear as sharp, intense spikes inthe spectra; whereas, the Doppler signal from the bulk flow appears asbroad, weaker peaks. Rejecting sharp spikes in the spectra by softwarefurther reduces the influence of surface flow on the velocitymeasurement.

As can be understood from the above description, the velocimeter of thisinvention has several advantages, such as: (1) not being subject toinaccuracies inherent in measurements of surface velocity, and ofdetecting a signal when there are few suitable reflectors on thesurface; (2) it avoids prolonged time periods during which datacollection cannot be made because of mode-hop laser instabilities; and(3) it can be used to confirm no flow (zero velocity) conditions bydistinguishing no flow conditions from a circuit break which otherwisewould result in no Doppler beat signal and thus no output signal due tothe AC coupling of the electronic circuitry.

Although a preferred embodiment of the invention has been described withsome particularity, it is to be understood that the invention may bepracticed other than as specifically described. Accordingly, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced other than as specifically described.

In the present disclosure, the methods disclosed may be implemented assets of instructions or software readable by a device. Further, it isunderstood that the specific order or hierarchy of steps in the methodsdisclosed are examples of exemplary approaches. Based upon designpreferences, it is understood that the specific order or hierarchy ofsteps in the method can be rearranged while remaining within thedisclosed subject matter. The accompanying method claims presentelements of the various steps in a sample order, and are not necessarilymeant to be limited to the specific order or hierarchy presented.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

What is claimed is:
 1. A method of determining the average flow of aflowing stream with a laser velocity meter comprising the steps ofrepeatedly changing the laser's position whereby the laser's position isrepeatedly changed with respects to a lens' position so that a laser'slight focus occurs at one of many points so that the velocity of thestream can be measured at several different points in the flowing streamto obtain a better estimate of average flow.
 2. A method in accordancewith claim 1 wherein the step of repeatedly changing the laser'sposition includes the step of repeatedly changing the laser's positionsideways with respect to a lens' position while moving it perpendicularto a beam path so that the laser's light focus occurs at one of manypoints along the width of the flowing stream, so that the velocity ofthe stream can be measured at several different points along the itswidth of the flowing stream, in order to obtain a better estimate ofaverage flow.
 3. A method in accordance with claim 1 wherein the step ofrepeatedly changing the laser's position includes the step of repeatedlychanging the laser's position along a light beam axis with respect tothe lens' position so that the laser's light focus occurs at one of manypoints along the light beam axis but at different depths in the flowingstream, so that the velocity of the stream can be measured at severaldifferent depths in the flowing stream, in order to obtain a betterestimate of average flow.